I think what I've written below is accurate:
Einstein proposed his theory of special relativity in 1915. The equations for general relativity, however, were published in 1917. Almost immediately, a Dutch astronomer named Willem De Sitter found that Einstein’s relativistic equations “predicted an exploding universe, in which the galaxies of the heavens moved rapidly away from one another” (Robert Jastrow, God and the Astronomers [New York: W. W. Norton, 1978]), 13. Another line of evidence supporting this conclusion had already been provided by Vesto Melvin Slipher in 1913. He observed that one dozen galaxies were in the Milky Way’s vicinity, “moving away from the earth at high speeds, ranging up to two million miles per hour. Slipher's hint was the first hint that the universe was expanding” (ibid). In the 1920s the expanding-universe concept gained momentum by means of further cosmological discoveries. Using a one-hundred inch telescope, Edwin Hubble and Milton Humason undertook monumental observations. See Brian Greene, The Fabric of the Cosmos: Space, Time, and the Texture of Reality (New York: Vintage Books, 2005), 229-233.